Solar cells or photovoltaic devices are well known and are made from various semiconductor materials. For example, approximately 18% efficiency of solar energy conversion has been achieved by using cyrstalline silicon (c-Si) material. Amorphous silicon (a-Si) is a new material which is being used for solar cell applications. The present efficiency of these cells, however, is rather low (6%). Despite this low efficiency, amorphous silicon is a particularly desirable semiconductor material for photovoltaic devices since a-Si has a bandgap (1.5-2.0 eV) in the range needed for high efficiency, is easy to deposit, has a high absorption coefficient whereby a thin film can absorb most of the light, and its electronic properties can be changed by doping with n and p type dopants.
A further approach taken heretofore in the photovoltaic art is to increase efficiency by using multiple gap devices which include, for example, plural cells between the transparent and opaque electrical contacts.